1. Field of the Invention
The present invention relates to a thermal head for a printer, and particularly to a thermal head suitable for high speed printing, which has a long-lived heater and a long-lived protective layer.
2. Description of the Prior Art
In the prior art, there are printers of an impact type, a thermal printing type, an ink jet type and the like. Among them, the impact type is most popularly utilized. However, a printer of the impact type has limitations in the number of dots printed per unit area and in the size of a single dot and is not suited for printing of fine characters. In addition, a printer of the impact type performs its printing operation mechanically and has the drawback that makes noise during operation.
For a printer of the thermal printing type, heater elements can be made very small since a thermal head can be manufactured by photolithography and therefore fine printing operation is possible. Such a printer of the thermal printing type performs printing operation thermally and does not produce any noise. In view of these merits, a demand for printers of the thermal printing type is rapidly increasing and it is desirable to make further improvements in the lifetime of a thermal head and the printing speed.
The performance of a thermal head depends definitely on the material of the heater and the material of a protective film applied thereon. In order to obtain a thermal head having excellent performance, it is necessary to develop appropriate materials for a heater and a protective film.
FIG. 1 is an enlarged fragmentary sectional view illustrating a conventional thermal head. A heater layer 12 is formed on a substrate 11 and lead wires 13a and 13b are formed on the heater layer 12. The heater layer 12 and the lead wires 13a and 13b are covered with an antioxidant layer 14 and an abrasion resisting layer 15.
In operation, the heater layer 12 generates heat between the lead wires 13a and 13b to which electric power is supplied. A thermosensible paper or an ink ribbon (not shown) is interposed between the thermal head and a platen (not shown) so that characters are printed on the thermosensible paper or transfer paper.
A conventional thermal head, e.g., as disclosed in Japanese Patent Publication No. 8234/1984, comprises a heater layer 12 of TaN, Ta-SiO.sub.2 or the like, an antioxidant layer 14 of SiO.sub.2 and an abrasion resisting layer 15 of Ta.sub.2 O.sub.5. Since the protective film of this thermal head is formed by two layers, namely, the antioxidant layer 14 and the abrasion resisting layer 15, the process of manufacturing the protective film is complicated and takes much time. In addition, although the combination of the SiO.sub.2 antioxidant layer 14 and the Ta.sub.2 O.sub.5 abrasion resisting layer 15 assures a thermal head having relatively long lifetime, further development is desired to obtain a thermal head having a longer lifetime and assuring higher printing speed with a considerable saving of energy.